PET generates images that represent the distribution of positron-emitting nuclides within the body of a patient. When a positron interacts with an electron by annihilation, the entire mass of a positron-electron pair is converted into two 511-keV photons (also referred to as 511-keV events). The photons are emitted in opposite directions along a line of response (LOR). The annihilation photons are detected by detectors that are placed on both sides of the LOR, in a configuration such as a detector ring. The detectors convert the incident photons into useful electrical signals that can be used for image formation. An image thus generated based on the acquired image data includes the annihilation photon detection information.
A large portion of the emitted photons is scattered (e.g., Compton scattering), before leaving the patient. In order to account for the scattered photons, scatter corrections may be performed using a single scatter simulation (SSS) method. The SSS model estimates scatter occurring within an axial field-of-view (FOV) of the scanner. However, scatter events may originate outside the axial FOV of the scanner. Estimating out-of-field (OOF) scatter may be difficult without knowing the emission activity outside the axial FOV. In certain types of imaging, such as in prospective scanning and reconstruction, the emission activity outside the axial FOV may not be known.